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Message from discussion State machine representation
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Dave Roberts  
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 More options Feb 1 2004, 1:44 pm
Newsgroups: comp.lang.lisp
From: Dave Roberts <ld...@re-move.droberts.com>
Date: Sun, 01 Feb 2004 18:44:19 GMT
Local: Sun, Feb 1 2004 1:44 pm
Subject: Re: State machine representation
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Kenny Tilton wrote:
>> 1. Just represent state machines simply with an integer or symbol state
>> variable and a case or cond form which switches on the current state. The
>> forms associated with any case/cond clause then determine the next state.

> That's what I have done with some simple machines. Symbols definitely,
> for readability, plus numbers would not work with CASE.

Right. Doesn't seem like it scales as well, however. Lots of top-level
namespace pollution. Probably the most simple way to get it done, however.
For simple programs, this seems like what I would use if I didn't have a
macro language built up to define state machines nicely (my eventual goal).

>> 2. Represent each state with a different function, bound at top-level.
>> The state machine can then be represented as a list or structure, with
>> those functions operating on it.

>> 3. Represent the state machine with a closure. Basically:
>> (defun make-state-machine ....
>>   (let ((statevar1)
>>         (statevar2)
>>         (current-state))
>>     (flet ((state1 (input) ...)
>>            (state2 (input) ...))
>>       (setq current-state state1)
>>       (lambda (input) (funcall current-state input)))))

> Not bad.

Thanks, I'm learning. I sense that closures are powerful constructs, but I'm
sort of struggling for how/when to use them. Typically, it seems like you
can always get something done another way, and so I struggle with when to
use that tool.

-- Dave


 
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